Effect of Meniscus Recession on the Effective Pore Radius and Capillary Pumping of Copper Metal Foams

Abstract

An experimental study is performed to characterize the effect of meniscus recession on the effective pore radius and capillary pumping of copper metal foams which are to be used as wicks in heat pipes for electronic cooling. Knowledge of the effective pore radius is critical in defining the capillary pumping of a wicking material, but is rarely measured under operating conditions. It is known that the meniscus of a liquid recedes when evaporating from a porous media, which could impact the effective pore radius and therefore the capillary pumping capabilities of the foam. To elucidate this impact, the evaporation rate is measured from foam strips wicking ethanol from a reservoir while applying heat fluxes to the foam. Using thermocouple and IR camera measurements, the measured evaporation rates are corrected to account for different thermal losses, including natural convection, direct thermal conduction to the liquid, and evaporation from the container. An analytical model is then developed to relate the evaporated mass to the maximum capillary pressure (minimum effective pore radius) provided by the foam. It is shown for the first time, that just before the onset of dryout, the recessed meniscus will lead to 15%, 28% and 52% decrease in effective pore radius for samples with 68%, 75% and 82% porosities respectively. The capillary pumping therefore increases during evaporation. This can have significant impact on the prediction of the capillary limits in two phase capillary driven devices.

Measurement of Capillary Radius and Contact Angle within Porous Media

“…In the present study, the effect of temperature on the capillary pressure (and effective capillary radius) can be neglected due to the small changes in temperature. As the heat input to the evaporator increases, evaporation causes the local menisci to recede into the wick . Just prior to dryout, the menisci in the evaporator recede to the base of the micropillars, resulting in the minimum capillary radius (and, hence, maximum capillary pressure).…”

Analysis of a Vertical Flat Heat Pipe Using Potassium Working Fluid and a Wick of Compressed Nickel Foam

“…A The theory of Shirazy and Fréchette [17], which says that the effective pore radius is decreasing when the wick is exposed to a heat flux, i.e., use of an effective pore radius obtained from an experiment without heat flux will underestimate the wick capacity. B…”

“…In general, from Equation (5), the heat transfer capacity will increase by decreased effective pore radius and/or increased permeability. Increased effective pore radius due to a heat flux, as described by Shirazy and Fréchette [17], will only have effect in the capillary pressure limited region. The Vapour static pressure limit Vapour static pressure limit high Vapour static pressure limit low Capillary pressure limit Capillary pressure limit high Capillary pressure limit low Limit of safe operation Safe operation Figure 6.…”

“…In general, from Equation (5), the heat transfer capacity will increase by decreased effective pore radius and/or increased permeability. Increased effective pore radius due to a heat flux, as described by Shirazy and Fréchette [17], will only have effect in the capillary pressure limited region. The experimental point above the vapor static pressure limit line can, therefore, only be explained if the real permeability of the wick was higher than the value (73ˆ10´1 2 m 2 ) used in the calculation of the corresponding limit.…”

“…Various methods exist for the determination of the permeability and the effective pore radius, and in most cases these properties are determined by use of a model fluid and without exposing the wick to a heat flux, for instance as in [16]. Shirazy and Fréchette [17] recently questioned the validity of the effective pore radius determined this way for the calculation of the wick heat transfer capacity (the capillary limit). They had experienced that wicks made of copper foam performed much better in heat pipes than predicted from existing correlations for the capillary limit.…”